There is a chemist (drugstore) near the fresh market at the main road, where all the tuktuk's wait. Another, bigger one is at opposite side of the Seven-Eleven at about the middle of the main road. The owner there speaks a bit English and is well equipped.
There are two Seven-Eleven supermarkets in Chiang Khong. One is near the fresh market where also the busses stop; the second is about one to two kilometers north at the right sight of the mainroad. Westerners tend very much to go to these western-styled supermarkets because that's what they are used to. More recommendable are the local shops, where you get almost everything you get in a Seven-Eleven; but there you support local families and not share-holders, and they are in many cases even cheaper. Don't worry too much about missing price-signs. When asking for the price, in absolutely the majority of cases the shop owner or employee will tell you the 'proper' price, what means that you pay the same price as locals do. Along the main road there are plenty of shops of all kind. Good and fresh food one gets on the fresh market and around.
There is an elder Lady on the fresh market, next to the Fresh Market Restaurant. She is sewing clothes of all kind.